1. Field of the Invention
The present invention relates generally to structural support systems in which high thermal gradients combine with high mechanical loads to potentially produce unacceptably high stress levels. In particular, the invention deals with the aft mounting system for transition duct members of cannular gas turbine combustion systems of advanced design. More specifically, the invention deals with the means of firmly attaching the transition duct member walls to structural members of the gas turbine without causing thermal or mechanical stresses in excess of those allowed by the materials.
2. Description of the Prior Art
In recent years advanced design gas turbine machines have been introduced to operate at increased turbine cycle temperatures to meet ever increasing load and efficiency requirements. These higher temperatures have combined with increased mechanical and aerodynamic loading to significantly increase the problems in obtaining long reliable life for those components directly subjected to the hot gas temperatures. In particular, in heavy duty gas turbines with cannular type combustion systems the transition duct members hereafter referred to as transition ducts or ducts, operate with wall temperatures approaching 1500.degree. F. while the surrounding machine structure and compressor discharge air is only at about 650.degree. F. This temperature difference tends to produce high thermal stresses particularly at the duct wall-to-support structure interfaces. Additional environmental factors such as combustion instability, machine vibration, buffeting due to external air flow, and the relatively high weight of the transition ducts impose large mechanical loads and resultant stresses in the aforementioned interface region between the duct wall and support structures.
The principal support for transition ducts in heavy-duty gas turbine combustion systems of the cannular type is at the aft (downstream) end of the ducts, just ahead of the first stage turbine nozzle. Axial, radial, circumferential, and torsional support are all required at the aft end. Conventionally, the required support has been provided by a series of flat plate gussets welded approximately perpendicular to the transition duct member outer wall, and then welded to a base plate which is in turn bolted to the first stage nozzle retaining ring structure. As turbine inlet temperatures have risen, it has been suggested to provide means to cool the transition duct wall-to-gusset joint to extend the capabilities of this simple, relatively inexpensive, support system. Such cooling has limited mitigating effects on the basic shortcomings of this support system.
Known support systems experience significant shortcomings due to concentrations of both mechanical and thermal stresses at the welded joint between the gusset(s) and the transition duct wall, particularly at the ends of this joint. The stresses result from the basic geometry of the interface. In addition, these high stresses occur where the material properties have been degraded by the weld process itself.
A further disadvantage of prior art systems is in the forward end to aft end radial support redundancy. Because of thermal growth differences and manufacturing tolerance stack-up between the forward and aft ends of the transition duct, there are additional built-in assembly and operational stresses to the forward and aft end support systems which further contribute to the low operational lifetimes for the effected parts.
A typical prior art gusset mount at the aft end of a gas turbine transition duct member is shown in U.S. Pat. No. 3,750,398 to Adelizzi. Therein, simple brackets (one for each transition duct) are secured by welding to the aft end of the transition duct, and are affixed by bolting to the machine inner casing. A somewhat similar weld/bolt transition duct member support is disclosed in U.S. Pat. No. 3,609,968 to Mierley, Sr., et al.
Prior patents showing the use of pivoting mounting devices to support aft portions of transition duct members include U.S. Pat. Nos. 3,481,146 to Jackson et al; 2,547,619 to Buckland; 2,529,958 to Owner et al; and 2,511,432 to Feilden. These patents are directed to providing retaining means which permit compensation for thermally induced displacements along various axes and generally recognize the need for avoiding excessive rigidity in combustion support systems.